diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,8 @@
 
 *   **v0.3.4.1**: Add in missing `Functor` and `Invariant` instances for
     `ProPre` and `ProPost`, as well as a bunch of instances for `ProPre`.
+*   **v0.3.4.2**: Add in missing `HFunctor`, `Inject`, `Interpret` instances
+    for `PostT`.
 
 Version 0.3.3.0
 ---------------
diff --git a/functor-combinators.cabal b/functor-combinators.cabal
--- a/functor-combinators.cabal
+++ b/functor-combinators.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 6600494526f505f4af6a0fd9727f0cea1be40aae42b1d8c0071419840e46406f
+-- hash: 30f4ef1eb5a260098c2bb6ed5f7db568e418a2254181c3d14ad09dea45de2798
 
 name:           functor-combinators
-version:        0.3.4.1
+version:        0.3.4.2
 synopsis:       Tools for functor combinator-based program design
 description:    Tools for working with /functor combinators/: types that take functors (or
                 other indexed types) and returns a new functor that "enhances" or "mixes"
diff --git a/src/Data/HFunctor/Route.hs b/src/Data/HFunctor/Route.hs
--- a/src/Data/HFunctor/Route.hs
+++ b/src/Data/HFunctor/Route.hs
@@ -185,6 +185,18 @@
                . contramap g
                . unPostT
 
+-- | @since 0.3.4.2
+instance HFunctor t => HFunctor (PostT t) where
+    hmap f = PostT . hmap (hmap f) . unPostT
+
+-- | @since 0.3.4.2
+instance Inject t => Inject (PostT t) where
+    inject = PostT . inject . (id :<$>:)
+
+-- | @since 0.3.4.2
+instance Interpret t f => Interpret (PostT t) f where
+    interpret f = interpret f . hmap getPost . unPostT
+
 -- | Run a @'PreT' t@ into a contravariant 'Divisible' context.  To run it
 -- in @t@s normal covariant context, use 'interpret'.
 --